This invention relates to zoom lens systems, and more particularly to a multilens basic objective of fixed focal length which constitutes a zoom lens system together with a front variable afocal attachment and which is suited for improving the Petzval sum of the entire system.
The conventional zoom lens systems having high relative apertures and large zoom ratios generally make use of a Gauss type image forming rear lens section or basic objective to allow for many advantages in compensating for the various residual aberrations. The Gauss-type basic objective has, however, a tendency to produce a Petzval sum of negative sign. As the zoom ratio is increased, or the minimum focal length is relatively shorter, therefore, the Petzval sum of the entire system often changes to a negative number with a high possibility of inclination of the ideal image plane in a positive sense. An additional disadvantage is that the curvatures of the refracting surfaces must be correspondingly intensified, thereby making it more difficult to manufacture a production run of lens elements economically while holding the lens parameters to specific values.
One of the present inventors has proposed in Japanese Laid-Open Patent Application No. Sho 51-99037 (U.S. Patent Application Ser. No. 655,542) a zoom lens having a high relative aperture and a zoom ratio of about 1:3 which comprises a positive focusing lens, a negative variator, a positive compensator and a positive image forming lens.